In order to extend the shelf life of a product, it is usual to heat-treat the product and its package. The selected level of heat-treatment depends, int. al., on the conditions under which the intention is to store the product packed in the package. A conventional method of carrying out shelf life-extending heat-treatment of a package and a food product filled into a package is so-called retorting.
Such a shelf life-extending heat-treatment of the packed food may suitably be put into effect using the method and under the conditions described in greater detail in International Patent Application carrying publication number WO98/16431 which is hereby incorporated as a reference. The packaging container is placed in a retort and heated therein with the aid of a first circulating gaseous medium, e.g. hot steam, to a temperature which in general lies within the range of 70-130° C. After a predetermined stay time at this selected temperature, the supply of the first gaseous medium is discontinued. Thereafter, the packaging container is cooled with a second circulating gaseous medium, for example cold air, and finally with a circulating liquid medium, for example cold water. The cooled packaging container is thereafter removed from the retort for further transport and handling. The total treatment time, including the time for heating up to and the time for cooling from the selected treatment temperature should be sufficient, in each individual case, to give the pertinent food a desired combination of a high F0-value and low C0-value. The expressions “F0-value” and “C0-value” are known to persons skilled in the art and related to the time (min.) which the food should need to be heated to a reference temperature (121° C.) to achieve the same level of sterility and the time the food would need, respectively, to be heated to a reference temperature (100° C.) to achieve the same level of cooking effect on all components of the food. It is obvious to a person skilled in the art that a higher treatment temperature in retorting gives a higher F0-value and a lower C0-value than a lower treatment temperature in a corresponding retorting during the same total treatment time, and that a retorting of the packed food should consequently be carried out at a relatively high treatment temperature within the range of 70-90° C. in order to achieve the desired combination of high F0-value and low C0-value.
Traditionally, this type of process is usually employed for packages of metal, glass or other materials possessing similar moisture barrier properties. Moreover, these packages are most generally relatively rigid, with the result that, during the retorting process, they are capable of withstanding quite powerful inner excess pressure from the product cooking in the closed package.
However, in recent times retorting of paper-based packaging laminate has been introduced. In order to withstand the retorting process, a number of variations of packaging laminates have been developed. One such packaging laminate is known from, for example, International Patent Application carrying publication number WO97/02140. The prior art packaging laminate has a rigid, but foldable core layer of paper or paperboard and outer, liquid-tight coatings of moisture and heat-resistant thermoplastic material on both sides of the core layer. In order to impart to the prior art packaging laminate tightness properties also against gases, in particular oxygen gas, the packaging laminate also displays a gas barrier, e.g. an aluminium foil, disposed between the core layer and the one outer coating.
From the prior art packaging laminate, retortable packaging containers are produced with the aid of packing machines of the type which, from a web or from prefabricated blanks of the packaging material, form, fill and seal finished packages in accordance with the so-called form/fill/seal technology.
From, for example, a flat folded tubular packaging blank of the prior art packaging laminate, retortable packaging containers are produced in that the packaging blank is first raised to an open, tubular packaging carton which is sealed at its one end by fold forming and sealing of the continuous, foldable end panels of the packaging carton for the formation of a substantially planar bottom seal. The packaging carton provided with a bottom is filled with the pertinent contents, for example a food, through its open end which is thereafter closed by an additional fold forming and sealing of the corresponding end panels of the packaging carton for the formation of a substantially planar top seal. The filled and sealed, normally parallelepipedic packaging container is thereafter ready for heat-treatment in order to impart to the packed food extended shelf life in the unopened packaging container.
However, it has proved that, in certain cases, problems may nevertheless arise in that the package absorbs liquid during the retorting process to such an extent that its mechanical properties are negatively affected. Above all, such problems arise at those portions where the packaging laminate displays open edges. These open edges are to be found in most parallelepipedic packages at a longitudinal joint which extends along the height of the package, and at both ends of the package. This problem is usually referred to as edge suction intake.
The above mentioned problem can, in certain cases, moreover be accentuated by the fact that paper-based packages often require a support pressure during the retorting process. The support pressure is the pressure which prevails in the retort and which balances the inner pressure which occurs because of the heating of the product in the closed package.